1. Field of the Invention
The present invention relates to the acquisition and processing of low frequency passive seismic data.
2. Description of the Related Art
There have been a number of attempts to collect seismic data in the low frequency range (0 to 5 Hz) with either active or passive sources. This data was thought to have potential in resolving problems in ways not available through conventional seismic methods. Among the potential areas considered, for example, were: lowering uncertainty levels in conventional seismic data; improving inversion results and resultant seismic images; providing data about full wave inversion (or FWI); and enabling better pre-stack depth imaging in areas with complex velocity structures.
In comparison with active seismic data, low frequency passive seismic signals originated from natural origins rather than conventional active seismic sources such as vibroseis or impulse sources. Low frequency passive seismic data could thus be acquired without the need to deploy active seismic sources(s) and thus cost much less. Low frequency passive seismic data has been deployed in some regions for use as direct hydrocarbon indicator tools.
However, so far as is known, high sensitivity, three-component direction or 3-C sensors were considered to be obligatory for low frequency passive seismic data acquisition. The three-component sensors were composed of transducers arranged in a common housing on mutually intersecting axes to sense the total vector of ground displacement. Built-in electronics were included in three-component sensors for preamplification of the electrical signals formed by the sensor transducers.
Although passive acquisition with 3-C sensors was less costly than active surveys, so far as is known there were limitations in use of passive 3-C sensors. The 3-C sensors with built in pre-amplifier electronics were expensive. The coverage area and density available was limited and sparse. There were also problems in identifying the type of seismic waves present in the data and in identifying the sources of the signals recorded.
Low sensitivity sensors were reported to be unsuitable for recording low frequency passive seismic data. This was particularly the case for what were known as uniaxial or single axis vertical sensors which had a single transducer which sensed movement along a single vertical axis.